1. Field of the Invention
The present invention relates to resin compositions for use in proofing, such as waterproofing, dust-proofing or rust-proofing, of steel plates having metal coating of zinc, aluminum or the like. More specifically, the present invention relates to aqueous resin compositions that are particularly suitable for filling or sealing up a clearance defined in a joint portion of steel plates forming a car body or the like, or as hemming sealers.
2. Description of the Related Art
Steel plates plated with zinc, aluminum or an alloy containing such a metal are used in a great quantity as incorporated in vehicles, building materials, electric household appliances and the like.
Water or dust is likely to collect in a clearance defined in a joint portion of such plated steel plates, or in a bent portion or a like portion of such a plated steel plate, and such collected water or dust causes the steel plates to rust.
In the case of a car body, for instance, a clearance defined in a joint portion of steel plates is filled with a resin composition so-called xe2x80x9cbody sealerxe2x80x9d in order to waterproof or dust-proof the steel plates. Various resin compositions each containing a polyvinyl chloride resin as a major ingredient are widely used as body sealer resin compositions (refer to Japanese Patent Applications No. HEI 4-80382 and No. HEI 7-345483).
Recent environmental issues, however, demand for resins replacing polyvinyl chloride resins containing chlorine or any plasticizer. Further, the recent trend toward improved working environments or the like demands for resin compositions not containing any organic solvent.
Aqueous emulsion type compositions are known as such resin compositions not containing any organic solvent. Most of such aqueous emulsion type resin compositions, however, are used as coated onto paper sheets or films. In this case, the viscosity of such a resin composition in a water-dispersed state is often intentionally lowered to ensure desired coating properties. Such resin compositions thus prepared are in no way suitable as sealers.
A surface-treated steel plate having a superior resistance to white rust has been proposed (refer to Japanese Patent Laid-Open Gazette No. 2000-000519). The surface treatment described in this Gazette comprises coating a steel plate plated with zinc or a zinc alloy or aluminum or an aluminum alloy with a protective film formed from a composition of which major components are a water-dispersible resin and/or a water-soluble resin and a polymer chelating agent having a chelate-forming group bonded to a polymer matrix. The protective film thus formed has a close tissue because the chelate-forming group contained in the protective film is adsorbed by the surface of the plating film.
The aforementioned surface treatment, however, is merely capable of forming a close protective film and hence cannot be expected to rust-proof a portion that is not coated with the film.
It is naturally conceivable to use an aqueous resin composition prepared by dispersing a resin in an aqueous dispersion medium without using any organic solvent as a sealer in attempt to lower the possibility of affecting the environment. Any one of known aqueous resin compositions, however, is not suitable for use as a sealer due to its very low viscosity.
Sealers are required to have a viscosity such as to allow relatively thick coating and good adherence to a subject to be sealed. Aqueous resin compositions, however, inherently have properties opposite to such requirements because they contain water in themselves. Further, where the subject to be sealed is a steel plate, aqueous resin compositions are more disadvantageous in terms of adherence because oil film is usually present on a surface of such a steel plate.
The inventors of the present invention have intensively studied in pursuit of an aqueous resin composition which is improved to overcome the foregoing inconveniences and made the present invention.
Accordingly, it is an object of the present invention to provide an aqueous resin composition for use as a sealer which is suited for waterproofing and rust-proofing of steel plates plated with zinc or a zinc alloy to be used in car bodies, building materials, electric household appliances or the like. The resin composition does not contain chlorine or any organic solvent and hence does not affect the environment while exhibiting a positive rust-proofing capability as well as water-proofing or dust-proofing capability.
According to the present invention, there is provided an aqueous resin composition for use as a sealer, comprising an aqueous resin as a major component, and a viscosity modifier adjusting the viscosity of the aqueous resin composition to a value ranging from 5,000 to 100,000 mPaxc2x7s, the aqueous resin being an aqueous emulsion polyolefin resin which comprises a polyolefin not containing chlorine as a constituent element while exhibiting a viscosity of 1,000 mPaxc2x7s or higher in a water-dispersed state having a solid resin content of 20% to 40% by weight.
In the above aqueous resin composition, the aqueous resin as a major component is an aqueous emulsion polyolefin resin which comprises a polyolefin not containing chlorine as a constituent element while exhibiting a viscosity of 1,000 mPaxc2x7s or higher in a water-dispersed state having a solid resin content of 20to 40% by weight, and the overall viscosity of the aqueous resin composition is adjusted to a value ranging from 5,000 to 100,000 mPaxc2x7s by the viscosity modifier. The composition thus constituted exhibits proper viscosity, heat resistance, durability and strength as a sealer. Further, since the composition does not contain chlorine or any organic solvent, it does not emit any harmful substance during preparation, coating and disposal thereof or on other occasions and hence will not raise any environmental pollution problem or like problems.
The aqueous resin composition for use as a sealer may further comprise a water-soluble magnesium compound in a proportion of 0.1% to 10% by weight.
When the water-soluble magnesium compound is added to the foregoing aqueous resin composition, the resulting composition has an advantage of remarkably rust-proofing a steel plate plated with zinc or a zinc alloy as well as the advantages described above.
These and other objects, features and attendant advantages of the present invention will become apparent from the reading of the following detailed description.
The present invention will now be described in detail.
The viscosity of the aqueous resin composition according to the present invention is adjusted to a value ranging from 5,000 to 100,000 mPaxc2x7s. If the viscosity of the composition is higher than 100,000 mPaxc2x7s, the coating property of the composition is unsatisfactory. If, on the other hand, the viscosity of the composition is lower than 5,000 mPaxc2x7s, the composition, if applied to a steel plate surface stained with oil, may flow down without fixing to a desired portion of the steel plate.
The aqueous emulsion polyolefin resin used as a major component in the aqueous resin composition of the present invention is preferably a dispersion of a particulate polyolefin having a particle diameter of several xcexcm in water.
The aqueous emulsion polyolefin resin exhibits a viscosity of 1,000 mPaxc2x7s or higher at normal temperature when the solid resin content thereof is 20% to 40% by weight. If the solid resin content is less than 20% by weight, the aqueous resin composition has a viscosity lower than the predetermined value and hence cannot serve as a sealer for sealing up a clearance defined, for example, in a joint portion of steel plates or in a bent portion of a steel plate. If the solid resin content is more than 40% by weight, the aqueous resin composition undesirably exhibits too high a viscosity to ensure a satisfactory coating property.
Though a viscosity of about 10,000 mPaxc2x7s is practical as the viscosity of the aqueous emulsion polyolefin resin, there is no particular upper limit of the viscosity.
Further, the aqueous emulsion polyolefin resin does not contain chlorine as a constituent element thereof.
Examples of polyolefin resins for use in the aqueous emulsion polyolefin resin include typical vinyl acetate polyolefins or their copolymers with other monomers, ionomers, and modified ionomers. Any appropriate one of these resins is selected according to the viscosity or adhesive property varying within the aforementioned range depending upon applications.
Examples of such aqueous emulsion polyolefin resins having a solid resin content and a viscosity within respective ranges specified above include commercially available products of MITSUI KAGAKU CO., LTD. in the name of xe2x80x9cCHEMIPEARL S100xe2x80x9d and xe2x80x9cCHEMIPEARL V200xe2x80x9d, both of which are registered trademarks. There is, however, no particular limitation to these products.
Usable as the viscosity modifier in the present invention is any appropriate inorganic filler or any appropriate organic thickening agent, or a mixture thereof.
As a matter of course, inorganic fillers for use in the present invention do not contain chlorine as a constituent element thereof. Examples of suitable inorganic fillers are inorganic compounds generally used in sealers, such as calcium carbonate, calcium silicate, bentonite, zeolite, talc, glass bead, and surface-modified titania; powdery metals; and fillers treated with organic substances. Any appropriate one of these inorganic fillers is selected according to the desired purposed. Among them, modified calcium carbonate and modified titania are preferable but there is no particular limitation thereto.
Some care should be taken in handling an inorganic filler in a fine powdery state or having a strong basicity because they may vigorously react with the resin or the water-soluble magnesium compound to be described later.
As a matter of course, organic thickening agents for use in the present invention do not contain chlorine as a constituent element thereof. Examples of preferable thickening agents are those generally known as thickening agents, such as polyethylene glycol (PEG), carboxymethylcellulose (CMC), hydroxymethylcellulose (HMC), cellulose powder, and sodium alginate. There is, however, no particular limitation to these thickening agents.
Addition of a water-soluble magnesium compound to the aqueous resin composition is preferable because it enhances the rust-proofing or corrosion-proofing action of the composition on steel plates plated with zinc or a zinc alloy and the like.
The water-soluble magnesium compound is used in an amount ranging between 0.1% by weight and 10% by weight. If the amount of the magnesium compound is less than 0.1% by weight, a desired corrosion-proofing action will not result. Addition of more than 10% by weight of the magnesium compound is not preferable because the corrosion-proofing action is not improved more than a certain level and, hence, an excess of the magnesium compound is wasteful and because the strength of the resin and the stability of the aqueous resin composition become lowered.
Examples of useful water-soluble magnesium compounds include magnesium carbonate, magnesium phosphate, and magnesium dihydrogen phosphate. Though these magnesium compounds are preferable in terms of their superior corrosion resistance and water-solubility, there is no particular limitation to these compounds.
In order to improve the properties of the aqueous resin composition, for example, the adherence to a steel plate, an aqueous adherent resin may be added to the aqueous resin forming the major component. In this case, the aqueous adherent resin is added so as not to vary the solids content of all the aqueous resins used in the composition. In other words, the aqueous adherent resin is added so that the total solids content of the aqueous adherent resin and the aqueous emulsion polyolefin resin is equal to the solids content of the aqueous resin used in the composition of the present invention.
Examples of such aqueous adherent resins include an aqueous urethane resin, an aqueous epoxy resin, and a mixture of these resins.
The term xe2x80x9caqueous urethane resinxe2x80x9d used herein is a general term which is meant to include aqueous dispersions of reaction products of polyisocyanate and active hydrogen containing compounds such as polyol. Examples of typical aqueous urethane resins include anion-type aqueous urethane resins prepared by addition of polyhydroxycarboxylic acid or amino sulfonate to isocyanate group, cation-type aqueous urethane resins, and nonion-type aqueous urethane resins. Thes e aqueous urethane resins may be emulsified or modified with acrylic, epoxy or alkyd resins.
Typical aqueous epoxy resins include those rendered water-soluble by introduction of carboxyl group, which may be further rendered highly functional by bonding an acrylic resin thereto.
Where the aqueous urethane resin or the aqueous epoxy resin is added alone, each of these resins is blended at a ratio (solids content ratio) of 30% to 70% by weight relative to 30% to 70% by weight of the aqueous emulsion polyolefin resin.
Where the aqueous urethane resin and the aqueous epoxy resin are added as a mixture, the mixing ratio (weight ratio) of the aqueous urethane resin to the aqueous epoxy resin ranges between 1:4 and 4:1.
If the proportion of the aqueous adherent resin in the mixture of the resins is less than 30% by weight, the resulting aqueous resin composition does not exhibit a satisfactory adherence. The aqueous adherent resin proportion of more than 70% by weight in the mixture of the resins is not preferable in terms of the resilience and impact resistance of the resulting resin composition.
The aqueous resin composition for use as a sealer according to the present invention may further comprise any one of appropriate known additives such as pigment, antioxidant, antifungal agent, mildewproofing agent, anti-foaming agent, and like additives.